Time scaling (e.g., time compression or expansion) of a digital audio signal changes the play rate of a recorded audio signal without altering the perceived pitch of the audio. Accordingly, a listener using a presentation system having time scaling capabilities can speed up the audio to more quickly receive information or slow down the audio to more slowly receive information, while the time scaling preserves the pitch of the original audio to make the information easier to listen to and understand. Ideally, a presentation system with time scaling capabilities should give the listener control of the play rate or time scale of a presentation so that the listener can select a rate that corresponds to the complexity of the information being presented and the amount of attention that the listener is devoting to the presentation.
A time scaling system generally requires a significant amount of processing power for real-time conversion of prerecorded digital audio data into a time-scaled digital audio data stream. Accordingly, many systems with limited processing power, such as portable telephones are unable to perform real-time time scaling of audio, and such systems must rely on a source that provides a time-scaled signal having the desired time scale. Further, user selection of the time scale requires that the source of time-scaled audio be able to promptly respond to a change in the selected time scale and provide the time-scaled audio data at the correct time scale.
U.S. patent application Ser. No. 09/849,719, entitled “Real-Time Control of Playback Rates in Presentations” describes a method for providing real-time control of play rates through a source having multiple channels of time-scaled digital audio, with each channel corresponding to a different time scale. Such methods allow transmission of the desired time-scaled data over a network to a receiver having low processing power. However, having many different time scales and associated audio data channels at the source requires a large data structure. Alternative methods that provide smaller data structures and still permit real-time time scaling in systems having low available processing power are sought.